Swift extension of Array<MutableCollection> won't allow reverse() - arrays

Question

I am attempting to extend Array<MutatingCollection> so I can mirror the contents of an Array of Arrays, but the compiler says I can't call reverse() on the elements in the array, despite reverse() being defined in MutatingCollection protocol.
I want to do something like this:
var table = [[0,1,2],
[3,4,5],
[6,7,8]]
table.mirror()
//table now [[2,1,0],
// [5,4,3],
// [8,7,6]]
Here is my (not working) code:
extension Array where Element == MutableCollection {
mutating func mirror() {
for index in self.indices {
self[index].reverse()
}
}
}
I have tried it as self.map {array in array.reverse()} as well (which I think does the same thing, but I don't fully grok map()) Both ways result in the same error message:
Member 'reverse' cannot be used on value of type 'MutableCollection'
Edit: I can call the same code directly and it works as I intended.
Playgrounds Screenshot
Perhaps I'm using extension improperly, or Swift Playgrounds is blocking my access somehow.

Answer 1

First of all, the extension should be declared like this:
extension Array where Element : MutableCollection {
You want to check that Element adheres to the protocol MutableCollection, not that it is a MutableCollection
However, then I'm not able to call the reverse method on the subscript for some reason. The best I've been able to do is this:
extension Array where Element : MutableCollection {
mutating func mirror() {
for index in self.indices {
self[index] = self[index].reversed() as! Element
}
}
}
Which works as you need it to work although the forced cast is very ugly and I dislike doing it. I suppose I should test the cast to be certain but I can't see any case where calling reversed() would result in a collection that couldn't be cast back to Element.
Edit:
I figured out the issue. The reverse() method is only valid on MutableCollection when it is also a BidirectionalCollection. This code now works correctly:
extension MutableCollection where
Iterator.Element : MutableCollection &
BidirectionalCollection,
Indices.Iterator.Element == Index {
mutating func mirror() {
for index in self.indices {
self[index].reverse()
}
}
}
Now the code should work for all MutableCollection whose elements are both a MutableCollection and BidirectionalCollection - such as [Array<Int>] or even [ArraySlice<Int>]
You can see the full code for reverse() in Swift 3.1 here:
Reverse.swift
extension MutableCollection where Self : BidirectionalCollection